Flight attendants are appealing to United States Congress to block a recent decision to allow small knives on commercial planes.

From April 25, passengers on US flights will be permitted to carry knives with blades up to six centimetres long.

While box cutters of the kind used by hijackers in the September 11 attacks remain banned, the decision by the Transport Security Administration (TSA) to relax the rules on pocket knives has appalled airline officials and staff.

Sara Nelson of the American Association of Flight Attendants says the policy has to be reversed.

"It's very simple for flight attendants, whose jobs changed on September 11th 2001, that we were not only aviation's first responders, but we became aviation's last line of defence," she said.

The TSA says easing the ban on knives will allow airport screeners to focus on "catastrophic" threats such as explosives.

TSA chief John Pistole said last weekend the new guidelines would bring US security regulations into line with international standards and were designed to help airport staff find more dangerous items.

"This is part of an overall risk-based security approach, which allows transportation security officers to better focus their efforts on finding higher-threat items, such as explosives," he said.

The shift comes against the backdrop of the US sequester crisis that has seen dramatic budget cuts force federal agencies to look at different ways of reducing costs.

The TSA has already frozen its hiring policy and eliminated overtime with homeland security secretary Janet Napolitano warning the sequester cuts could lead to a doubling of queues and waiting times.

"These items have been banned for more than 11 years and will add little value to the customer security process flow in relation to the additional risk for our cabin staff and customers," Delta Air Lines chief executive Richard Anderson wrote in a letter to the TSA.

"If the purpose is to increase security checkpoint flow, there are much more effective steps we can take together to streamline the security checkpoints with risk-based screening mechanisms."

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